how to implement QAbstractScrollArea

As your guys know, QScrollArea is simple and useful, but it can't always fullfil our requirement, so sometimes we need to customize our ScrollArea, QAbstractScrollArea is a good choice, here I only desribe some issues met about how to implement QAbstractScrollArea.

1. can't show viewport, you need to call the viewport's paint event in scrollArea's paint event.

2. if other functions still can't work, follow step 1.